Conveying device for magnetizable particles

ABSTRACT

An apparatus used in electrophotographic printing or copying for conveying magnetically attractable toner or developer particles that includes a stator, electric windings, and an end-plate made of magnetizable material arranged in parallel with the major dimension of the stator. The end-plate and the stator define a channel therebetween in which the particles are conveyed. Electric current supplied to the windings creates a magnetic field whose force moves the particles within the channel. This apparatus may be realized by converting a linear induction electromotor having a stator, an armature, and a translator and replacing the armature and translator with the magnetizable end-plate. Alternatively, this apparatus may be realized by converting a linear induction electromotor having a double stator and a translator and replacing one of the stators and the translator with the magnetizable end-plate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device for conveying magnetizableparticles from a first position to a second position by means of amoving magnetic field produced by a converted electromotor and moreparticularly to a device for the electrostatographic development of alatent image.

2. Description of the Related Art

The art of electrostatic printing or copying involves the formation ofan image by the steps of (i) applying on a dielectric an image-wisecharge distribution, also called latent image, and (ii) developing thelatent image, i.e. converting the latent image to a visible image bydepositing selectively light-absorbing particles, called tonerparticles, on the latent image. The image made visible as a result ofthe deposition of toner particles on the latent image that was presenton the dielectric is then transferred to a substrate and fixed on it toobtain the final print.

In electrophotographic printing or copying, a specific application ofelectrostatic printing or copying, the dielectric is a photoconductorand the image is formed by the steps of (i) uniformly charging aphotoconductor, (ii) image-wise discharging the uniformly chargedphotoconductor for obtaining thereby a latent image, and (iii)developing the latent image, i.e. converting the latent image to avisible image by depositing selectively light-absorbing particles,called toner particles, on the latent image. The image made visible bythe deposition of toner particles on the latent image that was presenton the photoconductor is then transferred to a substrate and fixed on itfor obtaining the final print.

In the application of toner development two processes are known, i.e.dry-powder development and liquid-dispersion development. The presentinvention relates to the dry-powder development. For further details onsuch dry-powder development reference is made to a.o. "IEEE Transactionson Electronic Devices", Vol. ED-19, No. 4, Apr. 1972, pp. 495-511.

According to the dry-powder process the toner particles are chargedtribo-electrically as a result of their being mixed with carrierparticles. A mixture comprising toner particles and carrier particles,possibly in admixture with further additives, is called developer.

Basically, xerographic copiers and printers 1 commonly use a developingsystem 2 with a magnetic brush 3 (see FIG. 1, which shows a schematicdiagram of a magnetic-brush developing unit as known in the art) fortransferring toner particles 4 from a supply 5 to a development zone 6on an image carrier, e.g. a semiconductor photoconductive drum 7. Onsuch a magnetic brush the developer particles commonly are provided asbristles on the surface of the outer sleeve. Said bristles aretransferring toner to the development zone 6 on the photoconductive drumcharged with a latent electrostatic image.

The amount of drawn particles 4 is controlled by a coating-thicknesscontrol means or doctor blade 13 before reaching the development zone 6.Part of the conveyed particles is picked up electrostatically by thephotoconductor 7, whereas the remaining particles continue to movethroughout the surface of the magnetic sleeve 8 and eventually return tothe supply holder 5 after having passed a scraping means or cleaningblade 14. Optionally, a mixing system 15, such as a rotating paddle, mayensure an intimate mixing of toner and carrier particles. To preventtoner exhaustion a feed system or toner hopper 16 is provided. e.g. witha feed roller 17. Optionally, a bias voltage 18 may be applied to thedeveloping system 2, e.g. to prevent fogging.

A variety of magnetic brushes is available, i.e. types with stationarymagnetic core and revolving outer sleeve, types with revolving magneticcore and stationary outer sleeve, or types with revolving magnetic coreand revolving outer sleeve. Developing systems with magnetic brushes aredescribed a.o. in "HITACHI components for electrophotographic printingsystems", pp. 5-11, published by HITACHI METALS INTERNATIONAL Ltd.,Purchase, 2400 Westchester Avenue, N. Y. 10577, U.S.A.

Due to mechanical restrictions the use of a similar magnetic brushsuffers from limitations of applicable speed and of technicalreliability.

A developing device that is not involving the use of moving permanentmagnets has been disclosed by Keefe and Yarmchuk under the heading"Non-mechanical printing toner transport system" in the IBM TechnicalDisclosure Bulletin, Vol. 26, No. 7A, Dec. 1983, pp. 3450-3451.Reference is made in that respect to FIG. 2, which is a schematicdiagram of a similar developing unit 20 with electrically alternatingfields according to the above-mentioned disclosure, two electricconductors 21, 22 being mounted perpendicularly to the requiredtranslation 25 of the toner particles 4 controlled by binary pulses 23,24 that are not concurrent in time.

OBJECT OF THE INVENTION

It is an object of the present invention to provide a device forconveying magnetically attractable particles from a first position to asecond position without involving the use of mechanically movingpermanent magnets.

It is a further object of the invention to provide an alternativedeveloping device with an accurate and reproducible control of thedevelopment, wherein the particles travelling path and travelling speedare dependent mainly on electrical parameters, and thus determined to alesser extent by auxiliary mechanical devices, and a higher reliabilityis obtained thereby.

Further objects and advantages of the present invention will becomeapparent from the detailed description following hereinbelow.

SUMMARY OF THE INVENTION

The objects of the present invention are realized by providing a devicefor conveying magnetically attractable particles from a first positionto a second position comprising a conveying means located between saidfirst position and said second position and containing a magnetic-fieldproducing means that is transferring said particles from said firstposition to said second position wherein said magnetic-field producingmeans is an asynchronous or induction electromotor.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be described in detail by way of illustrationreferring to accompanying diagrams, wherein

FIG. 1 is a schematic diagram of a magnetic-brush developing unit asknown in the art;

FIG. 2 is a schematic diagram of a developing unit with electricallyalternating fields as known in the art;

FIG. 3 is a schematic sectional view of a linear induction motor withsingle stator as known in the art;

FIG. 4 is a schematic sectional view of a linear induction motor withdouble stator as known in the art;

FIG. 5 is an exploded view of a linear motor with double stator as knownin the art;

FIG. 6.1 represents the magnetic flux applied by the windings of aconventional stator:

FIG. 6.2 represents the magnetic flux applied by the windings of astator fed by semiconductor diodes;

FIG. 7 shows a converted linear motor according to the presentinvention;

FIG. 8 shows a converted linear motor combined with a magnetic brushaccording to the present invention;

FIG. 9 shows an electrophotographic apparatus with a developing unitprovided with a converted electromotor according to the presentinvention;

FIG. 10.1 is a standard wiring diagram for a linear electromotor withdouble stator;

FIG. 10.2 is a wiring diagram for a linear electromotor with singlestator with provision for connecting control devices.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It will be understood that the present invention is not limited to thespecific embodiments described herein. Any person ordinarily skilled inthe art may disclose alternative embodiments and applications that areentirely based upon the principle of the present invention.

The present invention relates to a device for conveying magnetizableparticles (e.g. xerographic developer) from a first location or positionto a second location or position by means of a moving magnetic fieldproduced by a converted electromotor.

Basically, any electric motor may be built according to a rotary and alinear design. As it is assumed that the operation of a rotaryelectromotor is sufficiently known by those skilled in the art, thistype of design will not be illustrated into particulars. In the furtherdescription hereinbelow rather the surprising and advantageous use of aconverted induction electromotor as conveying device for magnetizableparticles will be explained.

Hereinbelow, a first device according to the present invention will bedescribed for conveying magnetically attractable particles from a firstposition to a second position, comprising a conveying means locatedbetween said first position and said second position, and containing amagnetic-field producing means (indicated with reference number 10 inFIG. 9, which will be described further hereinbelow) transferring saidparticles from said first position to said second position wherein saidmagnetic-field producing means is a converted asynchronous or inductionelectromotor. Since both terms, asynchronous electromotor and inductionelectromotor, are well-known synonyms, one single term will be used inthe further description, i.e. induction electromotor.

As a matter of fact, a first series of experiments have been conductedon the basis of a rotary induction electromotor of the type withexternal rotor. It should be mentioned that similar induction motorswith external rotor are rather frequently used in equipment for officeautomation, motion picture film, magnetic tape, metrology, telephony,etc.

A first series of experiments was e.g. conducted with a converted motormade by PAPST MOTOREN GmbH, Postfach 1435, D-7742 St.-GeorgenimSchwarzwald, Germany, of the type Auβerlaufermotor KM2.80 Q3-53.33.26.The conversion consisted mainly in first removing the external rotor andthen replacing it by a smooth plate, film or foil, or sleeve made ofnon-magnetizable and electrically non-conductive material, e.g. plastic.After connecting the stator to a three-phase power supply it appearedthat magnetizable particles in the vicinity of the stator are revolvingin the same direction as the magnetic field. The described conveyance ofmagnetizable particles could be further markedly improved by mountingaround the stator and spacing a few mm apart from it an end-plate madeof magnetizable material, such as e.g. iron or steel (in FIG. 9, whichwill be discussed further hereinbelow, such an end-plate is indicatedwith reference number 36).

Although the present invention may cover both rotary and linearelectromotors the further description will mainly focus on anelectromotor fit for rectilinear motions.

A linear induction motor is closely related to a rotary squirrelcageinduction motor. In this particular case the linear motor may berepresented as a rotary motor that was exploded and projected in aplane. Throughout the stator circumference a sliding magnetic field willbe created, also called travelling field (by analogy with rotary fieldin the case of the rotary motor).

The rotor thus becomes a rectilinearly moving translator, sometimescalled reaction rail. The stationary part of the motor is still calledstator (such as in the case of a rotary electric motor).

The translator currently is made of electrically conductive material.The variation of the magnetic field generated by the currents in thestator windings (also called stator coils) causes eddy currents to flowthrough the described translator. The magnetic forces on these eddycurrents produce the driving force.

Similar (unconverted) linear motors are rather frequently used in lineardrives, in conveying plants, railway traction, etc.

FIG. 3 is a schematic sectional view of a linear induction motor 30 withsingle stator as known in the art. The primary section 31 of the motoris provided with slots in which the electric windings 32 are mounted.The secondary section of the motor consists of a flat electric conductor33 made of non-magnetizable material (e.g. copper), which is locatedbetween the primary section 31 of the motor and an armature 34 of apermanent magnet. FIG. 4 is a schematic sectional view of a linearinduction motor 40 with double stator 31, 35 as known in the art. FIG. 5is an exploded view of a linear motor 40 with double stator 31, 35 asknown in the art.

Hereinbelow, a second device according to the present invention will bedescribed for conveying magnetically attractable particles from a firstposition to a second position comprising a conveying means locatedbetween said first position and said second position and containing amagnetic-field producing means that is transferring said particles fromsaid first position to said second position wherein said magnetic-fieldproducing means is an induction electromotor. Typically, this secondembodiment comprises a converted linear induction electromotor (in FIG.7, which will be discussed further hereinbelow such a magnetic-fieldproducing means is indicated with reference number 10, said firstposition with 11 and said second position with 12).

A second series of experiments has been conducted on the basis of aconverted linear induction electromotor. Although tests were carried outboth with a motor 30 of the type with single stator and with a motor 40of the type with double stator, the further description will focusmerely on a converted linear motor of the type with single stator.

The series of experiments was conducted a.o. with a converted motor ofthe type LMKK 14-3.2/314 manufactured by DEMAG MANNESMAN, Postfach 50 0325, D-22705 Hamburg, Germany.

In a first test arrangement included in the series of experimentstranslator 33 was exchanged for a protective covering film ofelectrically non-conductive and non-magnetic material (e.g. plastic)covered by magnetizable particles to be conveyed 4. The conveyanceresult was virtually zero and was evaluated with a classification figureof "0 to 10".

In a second test arrangement a magnetic film or foil (on a rubber base)the magnet poles of which exhibited a recurring mutual distance andwidth of 6 mm each was mounted at a distance of a few mm from thestator. As a result, on the surface of the stator the physical distancebetween the sequential windings, which were constituting electromagneticpoles, could be affected by permanent-magnet poles with a view to a morecontinuous distribution of flux around the stator. The conveyance resultof the present embodiment showed a slight improvement, but stillremained very poor and was evaluated with a classification figure of "3to 10". Accurate measurements of the magnetic-field intensities revealedthat an imbalance occurred between the positive and the negative fluxvariations around said magnet poles.

In a third test arrangement no magnetic film or foil was mounted, butinstead a semiconductor diode was installed in every phase of the motoror stator power supply. As a result, the cycles of the magnetic fluxapplied by the stator windings were rectified in a way that themagnetizable particles 4 were subjected to a uniform and more evenforce. A time interval of this flux is represented in FIG. 6.1 and 6.2,wherein FIG. 6.1 represents the magnetic flux applied by the windings ofa conventional stator and FIG. 6.2 the magnetic flux applied by thewindings of a stator fed by semiconductor diodes. The conveyance resultof the present third test arrangement was substantially better and wasevaluated with a classification figure of "6 to 10".

In a fourth test arrangement an end-plate 36 or foil made ofmagnetizable material, e.g. iron or steel, having a thickness ofapproximatively 1 mm was mounted at a distance of a few mm from thestator. In this case the space between end-plate 36 and stator 31 wasavailable for the magnetizable particles 4. FIG. 7 shows a similarconverted linear motor 70 according to the present invention. Theconveyance result improved up to a classification of "7 to 10". A fluxmeasurement using a HALL sensor recorded 3300 to 3960 Gauss (peakvalues), whereas the flux measuring result dropped to 2200 Gauss (peakvalues) if a PVC plate having a thickness of 2.5 mm was mounted betweenmotor and toner particles.

It should be mentioned that in each of the above-described embodimentsof the present invention said particles 4 may be magnetic tonerparticles or multicomponent magnetic developer particles. For furtherinformation on such particles and their application in a developingdevice reference is made a.o. to our EP-A 93,201,795.7-PCT-EP-94/01855.

In another embodiment of the present invention said first position 11comprises a feed system for supplying particles and said second position12 comprises a removal system for removing said particles. In a morespecific embodiment said feed system comprises a supply vessel and ametering means for controlling the amount of transferred particles.

In still another embodiment of the present invention said removal systemcomprises an electrostatic imaging element 7, preferably aphotoconductor, wherein the particles are transferred directly from theconveying device to the imaging element.

In still a further embodiment of the present invention said removalsystem comprises a magnetic brush transferring the removed particles toan electrostatic imaging element, preferably a photoconductor. FIG. 8shows a converted linear motor combined with a magnetic brush accordingto the present invention. In this FIG. but a few basic elements areindicated with reference numbers, such as magnetic brush 3, particles 4,supply 5, development zone 6, photoconductor 7, first position 11,second position 12, stator 31, stator windings 32, end-plate 36.Following typical benefits became apparent from the present embodiment :the possibility of vertical upward conveyance of magnetizable particlesand the practicability of (conventional) magnetic brushes on locationsdifficult of access.

A more integrated preferential embodiment of the present inventioncomprises an electrophotographic apparatus, e.g. copier or printer,having a developing device in accordance with any of the precedingdescriptions. FIG. 9 shows a similar electrophotographic apparatus 91with a developing unit 92 provided with a converted electromotor 93according to the present invention. It is assumed that an electrostaticlatent image has been formed on a photoconductor 7 that had beenpreviously charged and exposed (not represented in this FIG.).Photoconductor 7 is revolving in the direction indicated by arrow 9. Arotary conveying device 92 according to the present invention (inclusiveof an armature 36 for permanent magnet) is mounted in the proximity ofthe photoconductor 7 and is revolving in the opposite sense of rotation19.

As the design and the operation of a developing device with an inductionmotor as represented in FIG. 7 are very similar to those of a developingdevice with a magnetic brush as shown in FIG. 1, for clarity's sake inthe detailed description following hereinbelow similar component partsexhibiting a similar construction and a similar function are indicatedwith one and the same reference number.

The amount of drawn particles 4 is controlled by a coating-thicknesscontrol means or doctor blade 13 before reaching the development zone 6.Part of these particles is picked up electrostatically by thephotoconductor 7, whereas the remaining particles continue to movethroughout the surface of the motor and eventually return to the supplyholder 5 after having passed a scraping means or cleaning blade 14.Optionally, a mixing system 15, such as a rotating paddle, may ensure anintimate mixing of toner and carrier particles. To prevent tonerexhaustion a feed system or toner hopper 16 is provided, e.g. with afeed roller 17.

It should be mentioned that both the described rotary field in the caseof a rotary electromotor and the described travelling the field in caseof a linear electromotor may be generated by means of a three-phasepower voltage, as well as by means of a single-phase power voltage withan additional auxiliary condenser, as well as by means of an electroniccommutation. By way of example, FIG. 10.1 is a standard wiring diagramfor a linear electromotor with double stator 31,35 for a power supply bymeans of a three-phase voltage; when interchanging two connecting wires(see on the' right-hand side of the diagram) the direction of motion ofthe field and thus of the particles to be conveyed are altered. FIG.10.2 is a typical wiring diagram for a linear electromotor with singlestator the six ends of the three windings being arranged for receivingthe connection 37 of control devices.

Under given load conditions the speed of a rotary or linear inductionmotor is defined by the motor design and by the frequency of the powersupply the motor has been connected to. Consequently, a continuous speedcontrol is (but) possible by varying the frequency of the supplyvoltage, applied to the stator, by means of a frequency changer. On theone hand, the recent progress in the field of power electronics enabledthe manufacture of increasingly enhanced equipment for frequency controland correspondingly for speed control; on the other hand, the servicingexpenses for mechanical installations are increasingly rising.Therefore, a linear motor will now often constitute a useful alternativefor a regular motor in combination with a mechanical transmission.

Electrophotographic processes are suitable for use not only formonochrome or black-and-white images but also for polychromatic ormulticolor images. For the latter images several color separations canbe developed in sequence by using cyan, magenta, yellow and/or blacktoners. On occasion, colorless toners may be used also.

We claim:
 1. An apparatus for conveying magnetically attractable particles from a first position to a second position, comprising:a stator; an end-plate made of magnetizable material, said stator and said end-plate defining a channel therebetween; electric windings disposed within said stator; and means for supplying electric current to said windings to generate a magnetic field in said channel, said magnetic field applying a force to move the particles from said first position to said second position.
 2. An apparatus as in claim 1, wherein said particles are magnetic toner particles or magnetic developer particles.
 3. An apparatus as in claim 1, wherein said first position comprises a feed system for supplying said particles and said second position comprises a removal system for removing said particles.
 4. An apparatus as in claim 3, wherein said feed system comprises a supply vessel and a metering means for controlling the amount of transferred particles.
 5. An apparatus as in claim 3, wherein said removal system comprises an electrostatic imaging element.
 6. An apparatus as in claim 3, wherein said removal system comprises a magnetic brush that is arranged to transfer the removed particles to an electrostatic imaging element.
 7. An apparatus as in claim 5, wherein said imaging element comprises a photoconductor.
 8. An apparatus as in claim 6, wherein said imaging element comprises a photoconductor.
 9. An apparatus as in claim 1 wherein said stator and said end-plate are parallel and arranged linearly.
 10. An apparatus as in claim 1 wherein said stator is curved and said end-plate is at least partially concentric around said stator.
 11. Electrophotographic apparatus comprising:a stator; an end-plate made of magnetizable material, said stator and said end-plate defining a channel therebetween, through which magnetically attractable particles are conveyed; electric windings disposed within said stator; means for supplying electric current to said windings to generate a magnetic field in said channel, said magnetic field applying a force to move the particles from a first position to a second position; and means for forming said particles into an image. 